In order to improve the positioning accuracy of manipulators, particularly multiaxial industrial robots, in the past the aim has been to produce ever more accurate models of the manipulators. The parameters of such models are determined during a surveying or calibrating process, which is normally carried out once by the manipulator manufacturer. Typically this involves fixing to a hand flange of the robot an auxiliary means permitting a precise determination of the position, i.e. the location and orientation, normally referred to hereinafter as the “pose” of the flange in space. For example use can be made of reference plates having known features determinable by a camera or laser tracking system. Alternatively use is made of other known measuring systems, such as filament or wire measuring systems, etc.
As a result of regularly occurring imprecisions, e.g. elasticities of transmissions and structural elements of the robot, as well as a lack of dimensional stability thereof, the above-described, highly precise, external position measurement of the flange gives a different value compared with a parallel-performed, internal measurement of position values of the manipulator, e.g. by means of angle measuring means integrated into the joints thereof and which for pose determination are linked with a subsequent model calculation, the so-called forward transformation. The deviations derived from the thus determined position difference at the different locations of the working area of a manipulator are subsequently used for establishing a so-called “absolutely accurate robot model”, which is significantly more accurate than a theoretical “standard model” of the robot. This makes it possible to improve the absolute positioning accuracy of a multiaxial industrial robot from a few millimetres when using the standard model to less than one millimetre when using an absolutely accurate robot model.
In a parallel German patent application of the same applicant (“method and device for improving the positioning accuracy of a manipulator”; official filing number 102004026814.2), whose content is made into part of the disclosure of the present application, methods and devices for determining the above-described absolute accuracy of a multiaxial industrial robot are described. As a function of an external measuring system for minimizing deviations the manipulator is moved into an end pose essentially corresponding to a preset pose and subsequently internal position values of the manipulator are used in said end pose for parametrizing the absolutely accurate model. In this way it is possible to improve the positioning accuracy of manipulators on the basis of absolutely accurate control models for such a manipulator and in particular this makes possible the replacement of a random robot in a robot cell (working cell) by a different robot.
An important field of use of such absolutely accurate robots is cooperation between several robots. During such a cooperation two or more robots are simultaneously operated with respect to a workpiece, e.g. one robot holding and moving the workpiece, whilst the other robot carries out welding thereon. Alternatively it is e.g. possible for two robots to simultaneously transport an object, or one robot transfers to another robot an object during a movement. As it is assumed that an absolutely accurately measured robot can reach any point of its working area with a toleratable accuracy, it is regularly concluded that a measurement of a base coordinate system for one robot with respect to another robot is sufficient in order to allow a desired, cooperative movement.
However, it has been found that even the absolutely accurate measurement of robots is not generally sufficient in order to allow a problem-free use of several cooperating robots. The smallest deviations from the necessary tolerances e.g. give rise to large forces in a workpiece carried by two robots, the tools and on the actual robots and this can have serious damage consequences. One possible solution would be to compensate robot, tool and component tolerances by a flexible coupling of the tools, but this cannot be implemented for cost reasons. In addition, as a result of the ever more complex uses of cooperating robots by teaching (online programming) or afterteaching of an offline-produced robot control program, it is not possible to bring about a desired, exact program sequence in economic manner.
The problem of the invention is to permit an efficient operation of cooperating manipulators, such as multiaxial industrial robots, whilst avoiding the aforementioned disadvantages.